I have been fascinated by lower Tornillo Creek ever since my first trip down
Estufa Canyon and subsequently reading a paper by Margaret S Stevens and James
B. Stevens titled "Carnivora (Mammalia, Felidae, Canidae, and Mustelidae) from
the Earliest Hemphillian Screw Bean Local Fauna, Big Bend National Park,
Brewster County, Texas" in the Bulletin of the American Museum of Natural
History, 2003 (which can be found online, by the way). I'm not a
paleontologist by any means, but finding a fossil would be a treat: noting the
location and reporting it to park personnel. (No taking of fossils from
national parks, people! Leave them undisturbed and just take photos.) Then,
there are the rocks themselves. Estufa
Canyon is on the northern extent of
their study area, but I had a hard time trying to interpret the sediments I saw
there from their descriptions. Actually, their descriptions were very brief in
this paper, but they did publish a number of detailed
stratigraphic sections.
My main aim in exploring an area farther downstream Tornillo Creek from
Estufa Canyon, where a couple of these sections were constructed, was to see if
I could answer some of the questions I had concerning the sediments I observed
toward the eastern end of the canyon.

I began by hiking down the drainage that contains the Carlota Tenaja. The
picture below is of a small fold called an
anticline in the San
Vincente member of the Boquillas Formation of upper
Cretaceous age, composed
of sediments of lime and muddy lime deposited in an ancient tropical sea (as
Texas was in the tropics at that time). The thick white line marks approximately
the "axial plane" of the anticline, a plane which approximately divides the
fold into halves called "limbs". Note the thin white line. Imagine it to be the
surface above a large anticline for which you have no cross-sectional view as
you have here. You could still identify it by the fact the beds dip away from
the axial plane. Also note the oldest exposed beds would be in the center of
the anticline. Geologists can use these clues to map large anticlines that are
not exposed as this one is.

Very quickly walking down from the Old Ore Road, you come across Carlota
Tinaja - or, actually two tinajas: an upper and lower one. A tinaja is where
water action in a drainage has eroded out a deep cavity in which water can
collect. Tinajas are very important for wildlife in Big Bend. In the following
pictures you see the upper and lower tinajas, respectively. The rock here is
once again the San Vincente member of the Boquillas Formation. Note that the
tinajas are near the contact between thinly-bedded, shale-like rock like
that of the anticline above and more massive beds of
limestone. This is the
same situation as at the Ernst Tenaja, which
is located at the level of the contact between the thinly-bedded Del Rio Clay
overlying the massive Santa Elena Limestone. I'm wondering if this could be a
coincidence, or maybe there is something about such a contact that enhances the
chance a tinaja could form there.

The Boquillas Formation is noted for its fossils of Inoceramus clam,
and many specimens can be seen in the rocks around the Tinaja. One fairly nice
fossil can be seen in the next picture.

Apparently, geological field work has been done here. In the next picture
you can see drill holes where samples appear to have been taken for analysis.
If you look closely you can see three holes, all taken at different "horizons"
(stratigraphic levels).

As you hike farther down the drainage from Carlota Tinaja, the wash becomes
a small canyon with steep walls. On the way back I took the "high road" above
the canyon so that I could get an overview. In the following photo, looking
northeast, you can see several features. Roy's Peak owe's its existence to an
igneous
intrusion, magma that never reached the surface, called the McKinney Hills
intrusion. Since then, erosion has
removed the overburden of material, exposing the intrusion. Like several other
exposed intrusions that date about 32 million years ago during the
Oligocene epoch -
in the Grapevine Hills and the Rosillos Mountains - this was an intrusion of a
quartz-poor granite-like rock called
syenite. Also in the picture
is Alto Relex, a
horst, which is an
upfaulted block between normal
faults. This feature
was created during the
Basin and Range
faulting episode during the past 20 million years and is topped by the
lower Cretaceous Santa Elena
Limestone. The gray
sediments are of the Pen Formation of the upper Cretaceous, mostly lithified
deposits of clay with some sandstone and chalk. Although a marine deposit, this
sediment was indicative of a transition towards a terrestrial environment
as the land rose relative to sea level. The material in the foreground on which
I am walking and that resting on the Pen Formation is the
Miocene basin-fill
sediment in which I was particularly interested. Carlota Tinaja is close to
where, visually, the Old Ore Road and the canyon intersect in the picture.

The following was taken farther upstream at the canyon rim. Erosion has cut
down through the basin-fill into the Pen and Boquillas (tan-colored rock farther
up the canyon). Imagine that you are hiking down the drainage in this picture.
In doing so you are hiking "down-dip", meaning you encounter younger and
younger rocks as you proceed.

Hiking down-dip means you could come across exposed contacts between the
Boquillas and the Pen, and then between the Pen and the basin-fill deposits.
The first image below is of the Boquillas-Pen contact, marked by the long,
white line. The Pen is typically easily eroded and forms slopes rather than
shelves or cliffs. (In the Banta Shut-in area,
however, the alteration of the Pen by the heat of the McKinney Hills igneous
intrusion has turned it into a
metamorphic rock called
hornfels, and there it
forms steep cliffs bordering Tornillo Creek, narrowing down to the shut-in.) The
Pen is largely covered by debris and some vegetation here, but you can see the
characteristic gray color, labeled by "1" and Pen debris covering the Boquillas
("2"). There seems to be a fairly sharp contact between the two formations at
this location, but I've read where the contact is supposed to be gradational,
that is, with the Boquillas transitioning into the Pen. The contact does appear
to conformable, however, meaning the beds above and below the contact are
parallel, and little time (geologically speaking) has elapsed between the end
of the Boquillas deposition and the beginning of that of the Pen. The view is
roughly to the north and up-canyon.

The next contact you come to is where the Miocene basin-fill deposits lie
on top of the Pen Formation. The line of contact is along the white line on the
left of the image below. On the right of the image the contact is covered with
debris from the sediments above. This type of contact is called an
angular unconformity,
where the beds above and below the contact are not parallel. Typically, this
type of unconformity indicates a lot of time has transpired between the
deposition of the lower and upper sediments, in this case on the order of 100
million years. I take the discoloration as possibly the remnants of the soil
that was on the surface of the Pen before the Miocene basin-fill sediments
were deposited. If so, the ledge marked could be an ancient
caliche layer. Soils
"fossilized" in the geologic record are known as
paleosols The basin-fill
sediments are seen to consist of roughly stratified sand and well-rounded
gravel and cobbles, indicating they were deposited by water action. The second
image below is a close-up of the contact. The orangish color is either iron
oxide or iron hydroxide, or both. This raises the possibility that the soil
developed in a warm, humid climate, in which case the ledge mentioned above
would likely not be an ancient caliche.

Next, look at a close-up of the Miocene basin-fill sediments a bit farther
down the canyon. These sediments, lying right on top of the Pen Formation,
have been labeled the lower member of the Banta Shut-in Formation in the work
of Stevens and Stevens mentioned earlier. This formation is not yet officially
recognized by the geology community and is therefore considered "informal". In
this picture you can see
cross-bedding with beds
containing different sized clasts (sedimentary particles such as clay, silt,
sand, gravel, etc.), indicating the currents that deposited the sediment varied
in speed and direction. Gravel and cobbles indicate the stream flow was often
quite rapid.

Below, you can see what Stevens and Stevens call "Comanche Ridge",
although I have yet to see it labeled on a map. This was a major
destination of mine as the Stevenses published one of their stratigraphic
sections based on the rocks here. However, I was disappointed in that I had a
hard time seeing what they saw as indicated in their section. Most of the rocks
on the ridge were covered with debris. I could only find a few places where the
original beds could be viewed. Another puzzle is that their section is over 60
meters, and the topographic relief of the ridge is 40 meters or less. Since the
beds are nearly horizontal here (they dip less five degrees in a westerly
direction), topographic and stratographic relief should be nearly
the same. I'm thinking that they included observations from the other exposures
in the area besides the rocks on Comanche Ridge. Also, they no doubt had
substantial time available to find numerous exposures. And, of course, they are
professional research geologists, not a "geological tourist" like me. Note the
clouds building in the distance. This (April, 2015) was a wet period in Big
Bend and the desert was green and full of wild flowers. Also some striking
lightning displays at night while we were there. We got rained on three nights
out of five!

It was hard to find good exposures of rock on Comanche Ridge as so much of
the ridge was covered with debris. To make matters worse, for some reason a
number of photos I took were not recorded. I don't think I yet have the touch
for my wife's camera. I really liked my old Canon Elph; never had a problem with
it until it gave up the ghost. Plus, it had a view finder, not a screen that is
useless outdoors. Complaining over, here are some pictures that did come out.
First is one taken at the base of the ridge on the west side - a small exposure
of cross-bedded coarser sand over finer sand. Strangely, there was a scattering
of high-silica (as indicated by their angular
conchoidal
fracture) volcanic pieces of gravel to cobble size on the ground
in this area.The second is an exposure maybe
10 meters above the base of the ridge. This should be an exposure of part of the
informal Banta Shut-in Formation middle member of Stevens and Stevens in which
they found fossils of
Pseudaelurus,
the ancestor of saber-tooth tigers and today's cats (large and small). These
sediments are mostly clay, silt, and fine sand. If the ledges in the photo are
ancient caliche layers, they may indicate the former location of soils.

There were a lot of interesting fallen blocks around, but too bad you can't
easily tell what level they came from. The one in the picture below is
interesting because it is an example of graded bedding. This occurs when the
water carrying sediment slows down, depositing the heavier material first and
then ever lighter material until the flow stops.

From the top of Comanche Ridge, I took a panorama of vistas. The following
is looking east in the general direction of the canyon containing Carlota
Tinaja (labeled as "Canyon"). The top of Comanche Ridge should consist of the
middle member of the informal Banta Shut-in Formation. Here, it is a
well-bedded sandstone. The reddish sediments on both sides of Tornillo Creek
should be of the lower member of that formation. You can see grayish and
yellowish sediments on the opposite side of the creek that are likely the Pen
and Boquillas Formations, respectively. The notch in Cuesta Carlota labeled
"ET" is where Ernst Tinaja is located. A
cuesta is a ridge that is
steep on one side with a gradual slope on the other.

Looking in a northerly direction, you see Roy's Peak and Alto Relex, just
barely looming into view. I am not sure of the identification of the members of
the Banta Shut-in Formation, but it looks to me as if the middle member may be
the more resistant bedding and the lower member the slope-forming sediment.
This could well be revised, however. There are two other members of this
informal formation of Stevens and Stevens, the red clay member and the bench
member. Neither of these are mapped by Stevens and Stevens in this area of
Tornillo Creek.

The final vista I'll show is toward the northwest. The Chisos Mountains need
no introduction. The sediments marked as Miocene and
Pleistocene may not look
too exciting, but they have deep, scenic canyons (notably
Estufa Canyon) here and there. (The
Pleistocene epoch began about 2.5 million years ago and ended with the
termination of the last glaciation, about 12 thousand years ago.) The
vegetated floor is covered with recent sediments, younger than 12 thousand
years.

Heading north from Comanche Ridge, I wanted to check out the sediments lining
the west side of the creek and see if I could locate this desert spring that
appears on Roy's Peak Quadrangle. According to Stevens and Stevens, this spring
is due to a fault. This is reasonable, since a fault can interrupt the flow of
subsurface water, forcing it to the surface. In the picture below, taken on the
way to the spring, you see an outcrop with slope forming sediments (lower
member?) topped by more resistant sediments (middle member?).

The spring was easy to find, but I saw no evidence of a fault. This isn't
surprising, since the whole area is covered by recent material and any fault
would be buried out of sight beneath it. The new USGS map (Scientific
Investigations Map 3142, 2011) of Big Bend shows no fault at this location,
but there are undoubtedly many faults in the park that are not significant
enough to be on the map. The spring area is
heavily covered with vegetation such that the location of the source was
difficult. In the next photo you see that a watering tank for livestock was
built here with a pipe running from the spring to the tank. It was hard to get
a clear picture of both the tank and the pipe. The photo after that shows a
little water trickling out of the spring. Most of the water is still underground
as you can easily ascertain when your boots sink into the surface on the
Tornillo Creek side of the spring.

The final photograph below shows an outcrop of the lower member (?) of the
informal Banta Shut-in Formation along the west side of Tornillo Creek.

OK, why is this trip called "Abandoned!"? Luckily, I'm not
talking about myself. On the way back to Carlota Tinaja I spotted a
weird-appearing object on a small knoll on the east side of the creek. I was
getting rather tired at this point and almost blew it off, but curiosity got
the better of me, and I hiked over to see what it was. It was a backpack that
someone had abandoned years ago. I say "years" because it was faded and inside
was one of those 5-hour energy drinks with an expiration date of 2013. The pack
had "US" on the back and empty water bottles, but the strangest of the
contents were these two smart phones. I didn't want to see if I could haul the
pack out myself as I had my own stuff to carry, but I did take some things with
me, especially the phones that might help identify whom the pack belonged to. I
dropped those off at park headquarters. I also triangulated the position of the
pack (I really need to break down and buy a GPS) so park rangers could find it.
The problem there was that few good landmarks could be seen from that spot, but
I did the best I could. Hopefully, the pack is found so someone else doesn't
report a missing hiker. Likely that person got safely out of there, particularly
if the individual was part of a military exercise that I understand are often
held in the park. But... TWO smart phones? Left behind?